SQL的子查询与聚合函数（SQL Server）的

Question

我的设立是这样的：我有三个表 -

• 学生（StudentID，名字，姓氏等）
• StudentSemesters（StudentID，semid的等等），以及
• 学期（semid的，年）

我的要求是，让每一个学生，但只为自己的最后一个学期的细节。从逻辑上讲，这意味着该年度数字最高的学期。我似乎无法获得查询权。为了简单起见，“年”只是一个整数（例如2000年，1998年）。下面是我一直停留在一段时间当前查询：

SELECT dbo.Student.LastName + ' , ' + dbo.Student.FirstName AS Student, dbo.Student.Defence1Date, dbo.Student.Defence2Date, COUNT(StudentSemesters_1.SemID) 
      AS SemesterCount, dbo.Student.EntrySemester + ' - ' + 
       (SELECT dbo.StudentSemesters.SemID 
       FROM dbo.StudentSemesters INNER JOIN 
           dbo.ListSemesters ON dbo.StudentSemesters.SemID = dbo.ListSemesters.SemID 
       WHERE (dbo.Student.StudentCode = dbo.StudentSemesters.StudentCode) 
       GROUP BY dbo.StudentSemesters.SemID, dbo.ListSemesters.Year 
       HAVING (dbo.ListSemesters.Year = MAX(dbo.ListSemesters.Year))) AS Expr1 
FROM dbo.Student INNER JOIN 
      dbo.StudentSemesters AS StudentSemesters_1 ON dbo.Student.StudentCode = StudentSemesters_1.StudentCode 
GROUP BY dbo.Student.LastName, dbo.Student.FirstName, dbo.Student.Defence1Date, dbo.Student.Defence2Date, dbo.Student.EntrySemester, 
      dbo.Student.StudentCode 

Answer 1

你可以做到这一点，以获得详细信息只为每个学生最后一个学期：

SELECT 
    s.LastName + ' , ' + s.FirstName AS Student, 
    s.Defence1Date, 
    s.EntrySemester + ' - ' + s1.SemId AS Expr1 
FROM dbo.Student    AS s 
INNER JOIN dbo.StudentSemesters AS S1 ON s.StudentCode = S1.StudentCode 
INNER JOIN dbo.ListSemesters AS lm ON lm.SemID  = s1.SemId 
INNER JOIN 
(
    SELECT SemId, MAX(Year) AS MaxYear 
    FROM dbo.StudentSemesters 
    GROUP BY SemId 
) AS s2 ON s2.semId = lm.SemId AND ls.Year = s2.MaxYear; 

然而，如果你使用SQL Server 2005 +，你可以使用窗口函数来进行：

WITH CTE 
AS 
(
    SELECT 
     s.LastName + ' , ' + s.FirstName AS Student, 
     s.Defence1Date, 
     s.EntrySemester + ' - ' + s1.SemId AS Expr1, 
     ROW_NUMBER() OVER(PARTITION BY s1.SemId 
         ORDER BY Year DESC) AS RowNumber 
    FROM dbo.Student    AS s 
    INNER JOIN dbo.StudentSemesters AS S1 ON s.StudentCode = S1.StudentCode 
    INNER JOIN dbo.ListSemesters AS lm ON lm.SemID  = s1.SemId 
) 
SELECT 
    Student, 
    Defence1Date, 
    Expr1, 
FROM CTE 
WHERE RN = 1; 

但这不会得到SemesterCount，但你可能会这样做（这只是一个猜测）：

SELECT 
    s.LastName + ' , ' + s.FirstName AS Student, 
    s.Defence1Date, 
    s1.SemCount, 
    s.EntrySemester + ' - ' + s1.SemId AS Expr1 
FROM dbo.Student    AS s 
INNER JOIN 
(
    SELECT StudentCode, COUNT(SemId) AS SemCount 
    FROM dbo.StudentSemesters 
    GROUP BY StudentCode 
) AS S1 ON s.StudentCode = S1.StudentCode 
INNER JOIN dbo.ListSemesters AS lm ON lm.SemID  = s1.SemId 
INNER JOIN 
(
    SELECT SemId, MAX(Year) AS MaxYear 
    FROM dbo.StudentSemesters 
    GROUP BY SemId 
) AS s2 ON s2.semId = lm.SemId AND ls.Year = s2.MaxYear;